Coaxial cables are known per se, for example, from U.S. Pat. No. 4,368,576, filed by Applicants. The known coaxial cables usually comprise an elongated, central conductor of metal which is concentrically situated in an elongated, tubular outer conductor of metal. Said central conductor is usually composed of a solid copper wire which is circular in section. Copper-clad wires of aluminum or steel are also known to be used for this purpose. Central conductors composed of a bundle of stranded or wound wires, so-called litzes, are also known.
The outer conductor of a coaxial cable is often composed of a layer of fine, stranded or wound metal wires or a wound metal foil. Aluminum or copper, which latter material may be tin-plated or not, is usually used as the material for these wires and foils. An important property of stranded outer conductors is that they provide the coaxial cable with a high degree of flexibility.
The central conductor and the outer conductor are generally separated from each other by a layer of an electrically insulating material, preferably a solid or foamed synthetic material. Coaxial cables in which air is used as the electrically insulating material between the conductors (so-called "semi-air spaced cables") are also known.
If necessary, one or more additional protective coatings of an electrically insulating material, preferably a synthetic resin, can be provided on the outer conductor. Dependent upon the usage of the coaxial cable, these coatings are provided with reinforcing elements, for example in the form of wires of metal or synthetic resin which are wound in the same direction. The presence of such protective coatings, however, is not absolutely necessary. For example, it is known to use bundles of coaxial cables without a protective coating in transmission cables of ultrasound equipment.
The known coaxial cable has disadvantages. It has been found that when this type of cable is miniaturized, the provision of the outer conductor becomes problematic. This applies both to a stranded outer conductor and to an outer conductor of metal foil. For example, the metal wires used for a stranded outer conductor must have a minimum thickness. The use of wire thicknesses below 25 micrometers results in an unsatisfactory stranding process. In addition, also when larger wire thicknesses are used, the stranding process proceeds very slowly. When relatively thin coaxial cables with a stranded or wound outer conductor are used, the rates typically are of the order of 10-30 cm per minute. Also, when coaxial cables comprising an outer conductor which is made of a foil are miniaturized, production-technical problems occur when the foil is provided. In practice it has been found that it is impossible to wind the foil when the diameter of the cables is less than 1.5 mm. However, when thicker cables are used, the provision process is very laborious and time-consuming.